The invention relates to the noninvasive measurement of polymer composition by electronic absorption spectroscopy. In particular, the invention describes methods and devices for in-situ analysis of molten polycarbonate by a combination of UV/visible absorbance spectroscopy and multivariate data analysis.
The melt (LX) polymerization process utilizing bisphenol A (BPA) and diphenyl carbonate (DPC) is one of the most efficient non-phosgene routes of polycarbonate production. Still, the formation of Fries rearrangement products during melt polymerization can be problematic. Fries rearrangement products result from the conversion of phenolic esters into corresponding ortho and para hydroxyketones as a result of the inherent stability of polybenzenoid compounds. Polycarbonates produced by the melt process typically have higher Fries content than polycarbonates produced by the interfacial method. Excess Fries product can lead to differences in physical properties, such as flow and ductility, between polycarbonate produced by the melt process and polycarbonate produced by more traditional interfacial methods. It is important, therefore, to monitor and control for excess Fries produced during polymerization. In addition, in many cases it is also important to monitor the amount of xe2x80x9cuncappedxe2x80x9d polymer chains. Uncapped polymer chains are those chains which terminate in a free phenolic group, as opposed to being terminated with an aryl carbonyl group. It has been found that the hydrolytic stability of polycarbonate is inversely proportional to the amount of uncapped chain ends. Thus, a method which provides accurate analysis of Fries products and the amount of uncapped chain ends would be of value for the optimization of polymerization reaction conditions, both in the research setting and for on-line monitoring at the production scale.
Conventional techniques for monitoring Fries products generally involve analyzing aliquots from the reaction mixture by methods such as liquid chromatography (LC), or nuclear magnetic resonance (NMR). Similarly, techniques employed for the analysis of phenolic end-groups include IR spectroscopy, proton NMR, and potentiometric titration. These and other known methods of laboratory analysis, however, are time consuming and/or require relatively large sample sizes. Furthermore, these methods are not well-suited to on-line analysis of polycarbonate formed during large-scale polycarbonate production in that they require multiple sample preparation steps which are time-consuming, add to the overall error, are potentially dangerous at the high temperatures used for polymerization, and are not easily adaptable for remote monitoring using optical fibers. Also, removing aliquots may alter the reaction conditions or sample constitution, and provides only temporally discrete data points, rather than a continuous profile.
As an alternative to monitoring reactions during the polymerization, samples may be analyzed after the reaction is complete, and unsatisfactory products discarded. For example, a known technique for monitoring phenolic end-groups employs ultraviolet (UV) absorption spectroscopy to measure absorbance of phenolic end-groups at about 287 nm. Another technique for monitoring phenolic end-groups employs ratiometric ultraviolet absorption spectroscopy where absorbance of carbonate units in the spectral region of about 266 or 272 nm is compared to the absorbance of phenolic end-groups at about 287 nm. The measurements are typically performed by dissolving the polymer in a suitable solvent followed by UV spectrophotometric analysis or by a gel permeation chromatography and UV analysis (E. Shchori and J. E. McGrath, J. Appl. Polym. Sci., Appl. Polym. Symp., 34:103-117 (1978); C. O. Mork and D. B. Priddy, J. Appl. Polym. Sci., 45:435-442 (1992). Post-reaction sampling, however, does not enable real-time optimization of reaction parameters and, therefore, may result in the synthesis of a polymer batch of substantially inferior quality.
Thus, there is a need for an on-line method for optimization of production-scale polycarbonate synthesis. The method should eliminate the need for direct sampling and allow for the generation of continuous data. Also, the method should enable optimization of the overall melt polymerization process and improve plant capability. Similarly, there is a continuing need to evaluate economically superior reactant systems. Thus, the method should be adaptable to combinatorial (small-scale) evaluation of new reactant and catalyst combinations, as well as production-scale reactant systems.
The present invention relates to methods and devices for in-situ measurement of multiple reaction components of interest in molten polymer. The methods and devices described herein are suitable for measuring reaction components such as Fries products and uncapped phenolic end-groups in molten polycarbonate, in reactions ranging in size from small scale combinatorial formats to production-scale reactors.
Thus, in one aspect, the invention comprises an apparatus for the in situ monitoring of molten polymer and/or oligomer composition comprising: a light source; a fiber optic transmission probe, wherein the probe transmits at least one substantially monochromatic radiation from the light source to irradiate a sample comprising at least one polymer and/or oligomer and collects light transmitted from the irradiated sample; a spectrophotometer, wherein the spectrophotometer monitors radiation comprising UV/visible light absorbed by the irradiated sample; and a data analysis system, wherein the data analysis system correlates absorbance to at least one predetermined reaction component.
In another aspect, the present invention comprises a method for in situ monitoring of molten polymer and/or oligomer composition comprising the steps of: providing an optical contact between a fiber optic probe and a stream of a molten sample comprising at least one polymer and/or oligomer; irradiating the molten sample with at least one wavelength of substantially monochromatic radiation; monitoring UV/visible light adsorbed by the molten sample; and correlating the UV/visible light absorbed by the irradiated molten sample to levels of at least one reaction component of interest.